What is this weird ugly dark join using mirror modifier, smooth shading, and possibly subsurface modifier?
I should have watched further in the video I was following - there's an errant face in the join left over from extrusions which I needed to remove.



It looks like perhaps "Auto Smooth" selectively applies smooth shading to edges based on the angle between the faces they delineate, and, as the issue wouldn't show up with flat shading, the flat shading applying to the edges at the join was hiding the issue.  (I could be very wrong).